1. Field of the Invention
The invention relates to a sheet stacking apparatus capable of sorting each of a predetermined number of sheets and to a sheet processing apparatus as well as an image forming apparatus each provided with the sheet stacking apparatus.
2. Description of the Related Art
Recently, in an image forming apparatus such as a copying machine, a printer, a facsimile, and the like, there is known an inside discharging type image forming apparatus in which an image reading apparatus is disposed in an upper portion of an image forming apparatus main body and a sheet discharging portion for discharging a sheet is interposed between the image reading apparatus and the image forming apparatus main body. Further, as well as the inside discharging type, there is also known an image forming apparatus in which a sheet processing apparatus for executing a post process such as a staple process, a perforation process, and a sorting process to a sheet discharged to a sheet discharging portion is disposed upstream of the sheet discharging portion so that process performance is improved.
However, in the inside discharging type image forming apparatus, since the sheet discharging portion is disposed between the image reading apparatus and the image forming apparatus main body, when an discharged sheet is visually recognized, an operator must visually recognize his or her own sheet by looking into the sheet discharging portion. Further, when another sheet is discharged onto plural sorted sheets, since a sorted sheet is covered with the other sheet, the operator must visually recognize the sorted sheet by removing the other sheets, for example.
To cope with the problem, there is proposed an inside discharging type image forming apparatus which improves a visual recognition property of a sheet discharged to an discharging portion by discharging the sheet in a state that it is shifted a predetermined distance in a sheet width direction intersecting a sheet discharging direction every predetermined number of sheets (one set) (see Japanese Patent Laid-Open No. 2009-7151).
However, in the image forming apparatus described in Japanese Patent Laid-Open No. 2009-7151, a sheet is shifted and discharged using a reference line on a proximal side (side where the sheet is visually recognized) as a reference regardless of a sheet size. Accordingly, when, for example, an A3 size sheet is discharged after an A4 size sheet has been discharged, since the A4 sheet is covered with the A3 sheet, it becomes difficult for a user who has output the A4 sheet to visually recognize that the A4 sheet exists. Further, since in order to remove or dislocate the A3 sheet on the A4 sheet it is necessary to take out the A4 sheet, ease of sheet removal is also reduced. Furthermore, when, for example, the A3 sheet is in an aligned state, the A3 sheet becomes out of alignment when the user removes and dislocates the A3 sheet to take out the A4 sheet.
It is an advantage of the invention to provide a sheet stacking apparatus which ameliorates the visual recognition property problem and ease of removal of a discharged sheet and an image forming apparatus including the sheet stacking apparatus.